1. Field of the Invention
The invention relates to a circuit arrangement for limiting the power of the optical signal emitted by a laser, in which an operating voltage source, a laser diode and the parallel connection of a plurality of series circuits of a resistor with the main electrodes of a transistor form a circuit, and in which the same drive signal is applied at least indirectly to the control electrodes of all the transistors.
2. Description of the Related Art
Patent Abstracts of Japan disclose in the publication No. JP 570 10 279 a circuit arrangement in which an operating voltage source, a laser diode and the parallel connection of a plurality of series circuits of a resistor with the main electrodes of a transistor form a circuit. In this circuit arrangement, the same drive signal is applied to the control electrodes of all the transistors. In accordance with the document, this circuit arrangement solves the problem of a reduced power input with respect to a circuit arrangement having only one transistor. No information can be discerned on limiting the power of the optical signal output by a laser in the event of a fault.
For the purpose of transmitting messages, increasing use is being made in telecommunications engineering of glass fiber transmission systems in which message signals present as electric signals are converted with the aid of a laser into optical signals which are fed via an optical fiber to a receiving device where they are reconverted into electric signals.
A risk associated with the operation of such a transmission system comes where there is a breakage of the transmission link, which can result in the optical signal emerges from the optical fiber causing injury to the human eye. The maximum permissible irradiation (MPI) for the cornea according to the current regulations is reached an optical signal with a wavelength of 1,300 nm and an optical power of 3 mW when the emitting face of the single mode fiber from which the optical signal emerges is at a distance of 10 mm from the cornea. In the normal case, the transmission link is operated at power values of the optical signal lower than those specified above, with the result that no danger occurs in the event of breakage of the transmission link; in the event of a fault, however, power values which exceed the permissible limit value can occur in the optical signal.
In assessing a drive circuit for a laser, with respect to safeguarding against overshooting of the maximum permissible power of the optical signal, it is assumed that each component of the drive circuit can fail in such a way that the optical power output by the laser reaches a maximum value; only in the case of resistors is it assumed that a component failure always leads to an increase in resistance.
Circuits have become known in which the current through the laser diode is limited by a series circuit of a resistor with the main electrodes of a transistor. In such a circuit, the current is set by the laser diode via the control electrode of the transistor. A failure to be assumed taking account of the foregoing considerations consists in a short circuit of the main electrodes of the transistor, the resistance limiting the current through the laser diode and thus the power of the optical signal output by the laser diode. However, to achieve a prescribed power value of the optical signal emitted by the laser diode after aging of the laser diode and/or for operation of an uncooled laser diode at an increased operating temperature it is necessary to permit current values which in the case of a new laser diode and/or of operation at a lower operating temperature of the laser diode.
Another possibility for satisfying the maximum permissible irradiation (MPI) consists in lowering the coupling efficiency between the laser and the optical fiber to such an extent that the limit value for the maximum permissible irradiation upon breakage of the optical fiber is satisfied in the case of the maximum optical power output by the laser in the event of a fault. However, this measure contradicts the efforts to obtain a coupling efficiency between the laser and the optical fiber which is as high as possible.
It is known for transmission systems in which optical signals are transmitted bidirectionally via a single optical fiber to switch on a laser only if an optical signal has been received from the remote station; in the case of reception of an optical signal, it is assumed that the transmission link is fault-free. In addition to the limitation to bidirectional transmission systems which is occasioned by the principle, this outlay on circuitry and control required for its implementation is an evident disadvantage in the case of this measure.